There is no doubt any more: In order to mitigate global warming, the emission of greenhouse gases must be reduced, the sooner the better. This will then lead to a stabilization of the greenhouse gas concentration in the atmosphere - and in the very long term hopefully to a decreasing concentration.
The level at which the greenhouse gas concentration gets stabilized does determine the warming effect, i.e. the temperature increase of the earth's surface and of the oceans.
The following graph shows the relation between the greenhouse gas concentration (expressed as CO2-equivalents) and the resulting average global temperature increase on the surface. The data are drawn from AR4 WGI, Chapter 10.8. [i.e. from the IPCC Fourth Assessment Report, Working Group I]". The graph has been taken from Wikipedia.
The above graphs represents the state of knwledge according to IPCC as per November 2007. The black line in the middle of the range is the most likely relationship, the red line on top and the blue line on the bottom indicate the uncertainty (95% confidence interval). A temperature increase of more than about 2° C will with high likelyhood lead to dramatic effects on the environment. This is the reason why the European community suggests to limit the global warming to max. 2° C. This means according to the above graph limiting the greenhouse gas concentration in the atmosphere to about 450 ppm CO2 equivalents. The current value is about 380 ppm CO2.
Use our global warming simulation program (for Excel or OpenOffice) to simulate your own mitigation scenario - it is surprising to see the effects!